I. Field of the Invention
The present invention relates to communication systems, particularly to a demodulation element assignment method for a communication system capable of receiving multiple signals.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneously communication between a mobile station and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
A signal having traveled several distinct propagation paths is generated by the multipath characteristics of the cellular channel. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the mobile radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple path in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
The deleterious effects of fading can be mitigated by controlling transmitter power in the CDMA system. A system for base station and mobile station power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued Oct. 8, 1991, assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced communication with multiple base stations using a soft handoff process. A handoff process is disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Oct. 8, 1991, and assigned to the Assignee of the present invention.
In a cellular telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmitter power of each mobile station is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each mobile station may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a mobile station arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other mobile stations. On the other hand, if the mobile station transmitted signal is at a power level that is too high when received at the base station, communication with this particular mobile station is acceptable but this high power signal acts as interference to other mobile stations. This interference may adversely affect communications with other mobile stations.
Therefore to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each mobile station within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the ideal case, the total signal power received at the base station is equal to the nominal power received from each mobile station multiplied by the number of mobile stations transmitting within the coverage area of the base station plus the power received at the base station from mobile stations in the coverage area of neighboring base stations.
The path loss in the mobile radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the mobile station, operates on a different frequency than the reverse link, from the mobile station to the base station. However because the forward link and reverse link frequencies are within the same frequency band, a significant correlation between the average path loss of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each mobile station estimates the path loss of forward link based on the total power at the input to the mobile station. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the mobile station. From the estimate of the forward link path loss averaged over time, the mobile station sets the transmit level of the reverse link signal. Should the reverse link channel for one mobile station suddenly improve compared to the forward link channel for the same mobile station due to independent fading of the two channels, the signal as received at the base station from this mobile station would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus a rapid response of the mobile station transmit power to the sudden improvement in the channel would improve system performance.
Mobile station transmit power is also controlled by one or more base stations. Each base station with which the mobile unit is in communication measures the received signal strength from the mobile unit. The measured signal strength is compared to a desired signal strength level for that particular mobile station. A power adjustment command is generated by each base station and sent to the mobile unit on the forward link. In response to the base station power adjustment command, the mobile unit increases or decreases the mobile unit transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved.
When a mobile station is in communication with more than one base station, power adjustment commands are provided from each base station. The mobile station acts upon these multiple base station power control commands to avoid transmit power levels that may adversely interfere with other mobile station communications and yet provide sufficient power to support communication from the mobile station to at least one of the base stations. This power control mechanism is accomplished by having the mobile station increase its transmit signal level only if every base station with which the mobile station is in communication requests an increase in power level. The mobile station decreases its transmit signal level if any base station with which the mobile station is in communication requests that the power be decreased.
The existence of multipath can provide path diversity to a wideband spread spectrum system. A spread spectrum system generates a spread information signal by modulating an information signal with a pseudonoise (PN) code. Generally the PN code runs at many times the rate of the information signal. The rate that the PN code is generated is called the chip rate and the duration of one data bit of the PN code is called the chip time. If two or more paths are available with greater than chip time differential path delay, two or more processing elements, called demodulation elements, can be employed to separately demodulate these signals. These signals typically exhibit independence in multipath fading, i.e., they do not usually fade together. Therefore the output of the two or more demodulation elements can be combined to obtain path diversity. A loss of signal occurs only when the signals from all demodulation elements experience a fade at the same time. In an ideal system, both the base station and the mobile station employ multiple demodulation elements.
As a mobile station moves through the physical environment, the number of signal paths and the strength of the signals on these paths vary constantly, both as received at the mobile station and as received at the base station. Therefore, a receiver incorporating the present invention uses a special processing element, called a searcher element, that continually scans the channel in the time domain to determine the existence, time offset, and the signal strength of signals in the multiple path environment. The output of the searcher element provides the information for ensuring that the demodulation elements are tracking the most advantageous paths. The present invention provides a method of assigning the multiple demodulation elements to the multiple received signals based on the searcher element information.
In an exemplary CDMA cellular telephone system, each base station transmits a spread spectrum "pilot" reference signal. This pilot signal is used by the mobile stations to obtain initial system synchronization and to provide robust time, frequency, and phase tracking of the base station transmitted signals. The pilot signal transmitted by each base station in a system may use the same PN code but with a different code phase offset meaning that the PN codes transmitted by neighboring base stations are identical but skewed in time with respect to one another. Phase offset allows the pilot signals to be distinguished from one another according to the base station from which they originate. The mobile station's searcher element continues to scan the received signal at the code offsets corresponding to neighboring base station's transmitted pilot signals while in the call inactive mode. When a call is initiated, a PN code address is determined for use during this call. The code address may be either assigned by the base station or be determined by prearrangement based upon the identity of the mobile station. After a call is initiated the mobile station's searcher element continues to scan the pilot signal transmitted by neighboring base stations. When the pilot signal transmitted by a neighboring base station becomes strong enough to establish communication, the mobile station generates and transmits a control message to the base station currently servicing the call. The current base station provides the control message to the cellular system controller.
The cellular system controller begins the base station diversity or so-called "soft handoff" process. The cellular system controller begins by assigning a modem located in the new base station to the call. This modem is given the PN address associated with the call between the mobile station and the current base station modem. The new base station modem assigned to service the call searches for and finds the mobile station transmitted signal. The new base station modem also begins transmitting a forward link signal to the mobile station. The mobile station's searcher element searches for this forward link signal according to the signal information provided by the old base station. When the mobile station acquires the new base station modem transmitted signal, the mobile station may continue to communicate through the two base stations. Another base station could be added in the same manner as the first new base station above. In this case the mobile station may continue to communicate through three base stations. This process can continue until the mobile station is in communication with one base station for each demodulation element that the mobile station contains and beyond.
Diversity combining in the mobile station significantly advances the quality and reliability of communications in a cellular telephone system. A form of maximal ratio combining may be used to increase the benefit in which the signal-to-noise ratio is determined for each path. Each path may then be combined with the contributions from the other paths weighted according to the signal-to-noise ratio. Combining may be coherent because pilot signal demodulation allows the phase of each path to be determined.
In the path from the mobile station to the base station, path diversity reception is obtained in a similar manner. A base station may contain an analogous set of processing elements as the mobile station in that a searcher element may provide data to assign a plurality of demodulation elements. The present invention defines a method for assigning the demodulation elements to the multipath signals in the base station.
During communication with an end user, the demodulated data signals of a base stations are forwarded to the cellular system controller along with an indication of signal quality. The cellular system controller relays these signals to the end user. When a mobile station is in a base station diversity mode with two independent base stations, the demodulated data signals of both base stations are forwarded to the cellular system controller along with an indication of signal quality. The cellular system controller then combines the two versions of the mobile station signal or selects the signal with the best quality indication. An alternative system configuration may transmit the undecoded or even the undemodulated signals to the cellular system controller to allow a better diversity combining process to be used.
A typical base station configuration may contain multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. When a mobile station is in base station diversity mode and communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the cellular system controller. In fact, within a multi-sector base station, a system may be configured such that each demodulation element may be assigned to any arriving signal regardless of the sector that signal was received from. This system configuration allows a process called softer handoff and the present invention defines a method for assigning the demodulation elements for this configuration.
It is therefore the object of the present invention to provide a method of assigning multiple demodulation elements in a mobile station.
It is another object of the present invention to provide a method of assigning multiple demodulation elements in a base station.